This invention relates to electrohydraulic servo valves and control systems for the same.
Typical of such servo valves is the electrical linear force motor actuated hydraulic valve described in U.S. Pat. No. 4,040,445, issued to McCormick, incorporated herein by reference. Traditionally these servo valves have an electronically activated coil and armature that move back and forth linearly against the compliant force of a centering spring or bellows, similar to the operation of a loud speaker. The coil and armature on which it is wound is connected to a valve spool sliding within a valve spool housing. The valve is proportionally opened and closed in response to the electrical voltage applied to the coil, the voltage to valve operation being designed to maintain a linear relationship. The linearity of the coil motion versus applied voltage, and thus the proportional valve operation, is dependent on the linear relationship of the electromagnetic force generated by the coil working against the force of the centering springs, and the friction in the moving parts, especially the valve spool within the valve spool housing. To maintain a predetermined uniform relationship between electrical input and valve opening, the spring forces and mechanical tolerances of the moving parts must be tightly controlled, precisely adjusted and maintained over the life of the valve. These factors dramatically increase production costs while lowering repeatability and reliability. Additionally, the distance and bandwidth over which these counteracting forces are linear is small, therefore severely limiting the use of such valves.